[css3-syntax] unicode-range with only question marks, eg. U+???

Hi,

 From css3-fonts:

> Ranges specified with ‘?’ that lack an initial digit (e.g. "U+???")
> are also valid, and are treated as if there was a single 0 before the
> question marks (thus, "U+???" = "U+0???" = "U+0000-0FFF"). "U+??????"
> is not a syntax error, even though "U+0??????" would be.

As pointed out by John Daggett, U+??? is currently not tokenized as an 
unicode-range by css3-syntax, but should be.

Related issue: https://www.w3.org/Style/CSS/Tracker/issues/70

Proposed change, in §4.4.1. Data state:

> U+0055 LATIN CAPITAL LETTER U (U)
> U+0075 LATIN SMALL LETTER U (u)
> If the next 2 input character are U+002B PLUS SIGN (+) followed by a
> hex digit, […]

Replace "followed by a hex digit" with "followed by a hex digit or a 
U+003F QUESTION MARK (?) character".

-- 
Simon Sapin